1. Field of Invention
The invention relates to twin screw rotors for axis-parallel installation in displacement machines for compressible media, with asymmetrical transverse profiles with eccentric center of gravity position as well as number of wraps xe2x89xa72 and with pitch varying depending upon the wrapping angle (xcex1), which pitch increases in a first subdivision from the suction side screw end, reaches a maximal value at xcex1=0 after one wrap, decreases in a second subdivision until a minimal value, and is constant in a third subdivision.
2. Description of Related Art
Known from the publications SE 85331, DE 2434782, DE 2434784 are internal-axis, screw-type machines with non-constant pitch of the screw members or varying transverse profiles. The partially single-threaded inner rotor is balanced with the aid of counterweights. The construction expense necessary therefor is high and the assembly time-consuming. A further, general drawback compared to external-axis machines is the suction-side sealing, which cannot be eliminated.
Furthermore, described in the patent documents DE 2934065, DE2944714, DE 3332707 and AT 261792 are double-shaft compressors with screw-like rotors where rotors and/or housing are made up of disc sections of differing thickness and/or contour disposed axially behind one another, and thus cause an inner compression. Since defective chambers and eddy zones arise owing to the stepped construction, reduced efficiency results compared with screw rotors. Furthermore, problems are to be expected relating to shape retention during heating up in operation.
Screw-type compressors with outer engagement of the screw rotors, rotating in opposite directions, are represented by several publications:
DE 594691 describes a screw-type compressor with two outer meshing rotors running in opposite directions with variable pitch and thread depth as well as diameter variation. The profile is shown as single-threaded with trapezoid shape in the axial section. Indications about balancing are lacking, however.
DE 609405 describes pairs of screw members with variable pitch and thread depth for operation of compressors and decompressors in air cooling machines. A special transverse profile is not indicated, the optical impression suggesting a single-threaded trapezoidal axial section. There is no indication of balancing although operation is supposed to be at high rotational speeds.
DE 87 685 describes screw rotors with increasing pitch. They are intended for installation in machines for expanding gases or vapors. They are designed as single-threaded or multi-threaded screw members, there being no indication of balancing.
DE 4 445 958 describes a screw-type compressor with outer meshing screw elements, rotating in opposition, xe2x80x9cwhich become continuously smaller from the one axial end to the second axial end remote therefrom . . . xe2x80x9d They are used in vacuum pumps, motors or gas turbines. The profile is shown as a rectangular profile; proposed alternatively is an embodiment with a trapezoidal thread. Here, too, there is no indication of balancing.
EP 0 697 523 describes a compressor type with screw rotors with multi-threaded, outer meshing profiles and continuous change of pitch. The point symmetrical profiles (S.R.M. profiles) directly bring about a static and dynamic balancing.
Shown in EP 1 070 848 are screw-shaped profile bodies with variable pitch in two-threaded design xe2x80x9c . . . in order to be able to be better balanced.xe2x80x9d Lacking is the indication about a special profile geometry; the drawing shows a symmetrical rectangular profile in axial section.
In some of the previously known documents of the state of the art above, the outer diameters vary, which leads to problems in manufacture and assembly. Common to all the solutions proposed in the publications mentioned are the high leakage losses through use of unfavorable profiles: an axial sequence of well sealed working cells is not possible with such profiles; a good inner compression is not possible at low or medium rotational speeds (blow hole leads to vacuum losses and losses with respect to efficiency).
Profiles with good sealing off are disclosed in the printed publications GB 527339 (double-threaded, asymmetrical), GB 112104, GB 670395, EP 0 736 667, EP 0 866 918 (single-threaded).
According to the following two publications, single-threaded profiles with good sealing off are used. Their pitch varies, but the outer diameters are kept constant:
DE 19530662 discloses a screw-type suction pump with outer meshing screw elements, xe2x80x9cwhereby the pitch of the screw elements decreases continuously from their inlet end to their outlet end in order to bring about the compression of the gases to be delivered.xe2x80x9d The shape of the teeth of the screw rotor displays an epitrochoidal and/or Archimedian curve. The drawback of rotors of this kind is that the achievable inner compression is mediocre.
Proposed in WO 00/25004 are twin screw rotors, the pitch course of which is not monotone, but instead at first increasing, then afterwards decreasing, and finally remaining the same. The transverse profile is single-threaded and asymmetrical and displays a concave flank. The outer diameter is constant, a profile variation being possible.
In neither of the two aforementioned publications is the problem of balancing touched upon.
Disclosed in WO 00/47897 are multi-threaded twin delivery screw members with equal asymmetrical transverse profiles each with a cycloidal hollow flank, alternatively the pitch or the pitch and the transverse profile being able to be varied along the axis and xe2x80x9c . . . correspondence of profile center of gravity and point of rotation being achieved through respective design of the individual transverse profile delimitation curves.xe2x80x9d (=balancing). Provided in the screw interior (in the regions of the teeth) are screw-shaped channels which are intended to be passed through by a cooling medium.
A manufacturing limitation is the relationship thread depth/thread height, limited to values c/d less than 4, which leads to restriction of the compression rates achievable or to enlargement of construction space. The problem intensifies with increasing thread number. Moreover the manufacturing expense grows with increasing thread number, so that in principle single-threaded rotors would be desirable as long as the problem of balancing can then be solved satisfactorily and as long as multi-threaded rotors are not altogether more advantageous or necessary for other reasons (for example rotor cooling).
Described in the documents JP 62291486, WO 97/21925 and WO 98/11351 are methods for balancing single-threaded rotors, the pitches being presupposed as constant. With modified measures, similar methods can be used for balancing rotors with variable pitch, however with very severe limitation of the permissible geometry since a balancing through hollow spaces creates additional problems in casting, which become even greater because of the asymmetrical mass distribution as a condition of the pitch variation.
It is therefore the object of the present invention to propose technical solutions for balancing screw rotors with variable pitch and eccentric position of the transverse profile center of gravity, whereby the following requirements have to be fulfilled:
The object stated above is attained in that static and dynamic balancing is achieved with the twin screw rotors through calculated balancing of overall wrapping angle, defined pitch course and ratio of maximal pitch to minimal pitch, or is achieved at least 80% and is supplemented by changes in the geometry in the region of the screw ends.
The useful shortening of the screw spiral flanks coming to a sharp edge takes place along with coordination with a wrapping angle enlargement on both sides (xcexc) and with the pitch. Recesses in the region of the screw end faces are used as additional measures for the balancing, if extreme conditions require this.
Such rotors offer the best prerequisites for reduction of the energy requirement, the temperature, the construction size and the costs, as well as for a free selection of working materials in applications in chemistry and semiconductor technology. The following calculations give the theoretical bases, which show that a screw rotor according to the present invention fulfils the balancing requirement on the basis of its shape.
Special embodiments of the twin screw rotors according to the invention are described in the dependent claims.